Recently, silicon carbide (SiC) and gallium nitride (GaN) attract attention as wide-gap semiconductor elements. Those materials have high breakdown voltage strength which is ten times as high as that of Si, and a drift layer for ensuring breakdown voltage can become thinner to nearly one-tenth, thereby making it possible to reduce voltage when a power device is turned on. By doing so, even in a high breakdown voltage area which allows only bipolar elements to be used with regard to Si, unipolar elements can be used with regard to wide-gap semiconductor elements made of SiC and the like.
In a power semiconductor module used for an inverter circuit, a free wheeling diode is connected in parallel to a switching device. A Si-PiN diode is used as a free wheeling diode in a conventional power semiconductor module. The Si-PiN diode is a bipolar-type semiconductor element which is constructed such that voltage drop is reduced due to conductivity modulation when large forward-bias current is applied. However, the PiN diode has characteristics in that during the process from the forward bias state to the reverse bias state, a carrier which remains on the PiN diode due to conductivity modulation is turned into reverse recovery current. In a Si-PiN diode, reverse recovery current is large because life time of the remaining carrier is long. For this reason, there are problems in that the reverse recovery current increases turn-on loss and reverse recovery loss (Err) generated on the element at the time of the reverse recovery of the diode.
On the other hand, a schottky barrier diode (SBD) is a unipolar-type semiconductor element which generates almost no carriers due to conductivity modulation. Accordingly, when a schottky barrier diode is used for an inverter circuit, because reverse recovery current is very small, it is possible to keep the turn-on loss and the reverse recovery loss small. Since conventional Si has low breakdown field intensity, when an SBD is made with high breakdown voltage, high resistance is generated when electricity is applied. For this reason, the breakdown voltage of a Si-SBD is limited to approximately 200 V. However, because SiC has high breakdown field intensity ten times of that of Si, practical application of high breakdown voltage SBD becomes possible, and it is widely known that turn-on loss (Eon) and reverse recovery loss (Err) generated on the element at the time of the reverse recovery of the diode can be reduced.
Furthermore, in the main circuit of an inverter of a power module which uses a conventional Si-PiN diode, commutation surge voltage (ΔVp=L×reverse recovery di/dt) is added according to the product of the current change (reverse recovery di/dt) at the attenuation of reverse recovery current of a PiN diode and the main circuit inductance L. And, when the sum (E+ΔVp) of power supply voltage (E) and surge voltage (ΔVp) exceeds a breakdown voltage of the power semiconductor switching element, there is a possibility that the power semiconductor element may become broken. For this reason, various kinds of technologies to reduce inductance of the main circuit have been proposed.
Furthermore, in a gate drive circuit of a power semiconductor, it is well-known that gate resistance is made large in order to decrease current change (di/dt) at the attenuation of reverse recovery current of the PiN diode.
Non-patent literature: “The element marginal loss analysis method for high power density power converter by the Si-MOSFET/SiC-SBD pair” Reference by the Institute of Electrical Engineers of Japan, Oct. 27, 2005, Electronic device and semiconductor power converter joint study group EDD-05-46 (SPC-05-71)